The runaway success of Plants vs. Zombies has revived the tower defense genre in recent years and spawned a host of games that look to replicate its magic. Plenty of knockoffs—and even a few worthwhile titles—have challenged Popcap's blockbuster, but Robot Entertainment has been able to capture the casual game charm of PvZ with Orcs Must Die!, a $10 title that takes the conventions of current tower defense games and adds a first-person-shooter twist.

This story is simple but remarkably amusing and effective. You are the overconfident and undertrained apprentice of a war mage who has sworn to protect a series of mystic gates from a seemingly endless invading army of orcs and other beastly enemies. The game opens with your teacher's untimely death at the hands of a slippery set of stairs and you are left to fill in his shoes, a task for which you are eager, but unworthy. Can you prove to the old man that you have what it takes? It's a breath of fresh air to play a clueless schlep, and I found myself smiling or laughing whenever my budding war mage had something smart-mouthed to share.

This fresh take on the proceedings includes the game mechanics as well. Eschewing the common overhead view and forced perspective, Orcs Must Die sports a full 3D interface which you experience from your character's POV. Rather than representing an omniscient third party or an offscreen goal, you are onscreen and vulnerable at all times, and can move around and attack the invaders directly.

You're given a budget at the start of each level to spend on countermeasures to stop your unwanted green guests. These range from redoubtable spike traps to elaborate lighting zappers and even fellow defenders such as archers and knights. Many of these stack for added effect; for example; arrow walls and tar pits down a corridor are a cheap and effective early game combo, with the arrows pushing the hapless hellspawn into the tarry depths. You can lay out traps at your leisure before you open the doors to the orcs, but after the invasion begins you have only timed pauses in between most waves of attack to add traps or alter your strategies.

Being onscreen during the attacks is also an advantage–you are easily the most powerful countermeasure against the orcs on any given map, and you can make yourself more dangerous by improving or changing your weapons with the budget you're given to implement traps. More than once I found myself fighting through a failed defensive strategy and getting reasonable scores by jumping into the fray and delivering a magical smackdown on the invading horde. Although this is not generally a recipe for success, it's nice to have the option and it adds a somewhat elective element of twitch-based excitement to a genre that's more known for strategy than thrills. Dying won't end the game either, as you'll respawn at the mystic gate, but it's likely to ruin the time/goal metrics for the level you're playing in, causing you to try again with a more effective set of traps and tactics.

Games that mix genres effectively don't come along all that often, and Orcs Must Die is a textbook example of the peanut-butter-and-chocolate effect all designers hope to accomplish when they turn on the idea blender. People who have problems with shooters and more action-oriented strategy titles aren't likely to bite, but just about everyone else should find something to like here. Excitement starts to peter out towards the end of the game as play becomes more repetitive, but by that point you've already been served up dozens of hours of entertainment. Given the $10 price, it's an easy recommendation for me. There is also a sequel, the imaginatively named Orcs Must Die 2!

Note: The "Buy it" button on the Product Information page takes you to the vendor's site, where you can download the latest version of the software.